1. Field of the Invention
The present invention relates to a reducing gas generator using a solid reductant as a generation source of reducing gas such as ammonia that reduces nitrogen oxide in exhaust gas and to a solid reductant SCR system having the reducing gas generator.
2. Description of Related Art
An SCR (selective catalytic reduction) system is of use in a whole range of fields as a means for removing nitrogen oxide in exhaust gas. In the SCR system, nitrogen oxide in exhaust gas reacts selectively with reducing gas such as ammonia by catalysis and is thereby reduced to harmless nitrogen. When nitrogen oxide in exhaust gas discharged from a large-scale plant such as a thermal power plant is treated, ammonia stored in a high-pressure tank is directly conducted to the SCR system. When nitrogen oxide in exhaust gas discharged from a small-scale internal-combustion engine such as an automotive engine is treated, an ammonia aqueous solution or urea aqueous solution is used as an ammonia generation source which is installed in a vehicle and safe and secure in handling, as described, for example, in JP2005-282413A corresponding to US2007/0035832A1.
However, when the above aqueous solutions are used, their volumetric efficiency is low. Ammonia that is used effectively for exhaust treatment corresponds to about 30 percent of the volume of the aqueous solution, since solubility of ammonia or urea is limited. Thus, a large amount of aqueous solution is difficult to install in a limited space of the vehicle, and consequently practical realization of the SCR system that treats exhaust gas discharged, for example, from a small-scale automotive engine, is hindered. In addition, there is concern about aqueous solution freeze-up in a low-temperature environment, block in pipes with crystals precipitated out of the aqueous solution, and corroded pipes due to a corrosive aqueous solution.
Accordingly, a solid urea SCR system is proposed in which ammonia gas is conducted into an upstream side of an SCR catalyst disposed in an exhaust flow passage to reduce nitrogen oxide in exhaust gas. The ammonia gas is generated by heating and decomposing solid urea such as ammonium carbamate as an alternative ammonia generation source to the ammonia aqueous solution or urea aqueous solution, as described, for example, in Michael Kruger, et.al, “A Compact Solid SCR System”, MTZ worldwide June 2003 vol. 64 pp. 14-17.
In a conventional solid urea SCR system, as shown in FIG. 7, a liquid heating medium 209 such as heat transfer oil heated by a heat exchanger 201 is injected through an injection nozzle disposed under a solid reductant (urea) 208. As a result, a part of the surface of solid urea is instantaneously heated to have sublimation temperature or higher and accordingly a gaseous reductant 207 including a mixture gas of carbon dioxide and ammonia is generated by thermal decomposition. Consequently, the pressure in a space near a lower portion of the solid urea in a container 202 increases. The gaseous reductant 207 is supplied to an upstream side of an SCR catalyst 204 by controlling an amount of the supplied gaseous reductant 207, which is suited to an amount of nitrogen oxide in exhaust gas, through the detection of the above pressure using a pressure sensor 203.
Nevertheless, since a liquid heating medium is used for heating and decomposing the solid urea 208 in the conventional solid urea SCR system, the system requires not only the heat exchanger 201 for heating the solid urea 208, but also a circulation unit, which includes the injection nozzle, an oil tank, and an oil pump 205. The circulation unit injects the heating medium into the solid urea 208, recovers and circulates the heating medium. Furthermore, a mist of the heating medium is mixed in the gaseous reductant 207 and thereby the SCR catalyst 204 may be contaminated. Accordingly, a filter for separating the mist of the heating medium, for example, needs to be provided in a reductant feed pipe 206. Additionally, because the lower surface of the solid urea 208 is locally decomposed by the injection of the heating medium, the lower surface of the solid urea 208 includes both decomposed and undecomposed regions, so that a generation amount of the gaseous reductant 207 may be difficult to control. Also, the solid urea 208 is dissolved in the heating medium, and the solid urea 208 may be precipitated out of the dissolution in a circulation pathway of the heating medium to clog the circulation pathway. Therefore, a reducing gas generator using the solid reductant in the conventional SCR system has a complex structure, and the generation amount of the gaseous reductant 207 is difficult to stabilize.